1. Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically, to a liquid crystal display device that includes a backlight.
2. Description of the Related Art
Color display devices such as color television sets and color monitors usually express colors by additive color mixing of primary colors, R, G, and B (red, green, and blue). In color liquid crystal display devices, each pixel has a red sub-pixel, a green sub-pixel, and a blue sub-pixel which correspond to the primary colors R, G, and B, respectively. The luminances of the red, green, and blue sub-pixels are varied to express a diversity of colors. The red, green, and blue sub-pixels are realized by forming three sub-pixel regions within a single pixel region in a color filter.
Backlights in conventional liquid crystal display devices have a spectrum as the one illustrated in FIG. 31, and color filter elements which correspond to sub-pixels in conventional liquid crystal display devices have transmittances such as the ones illustrated in FIG. 32. In FIG. 32, R, G, and B represent, respectively, transmittances at which color filter elements of red, green, and blue sub-pixels transmit light of varying wavelengths. Liquid crystal display devices display an image (or the like) using light of a given spectrum that is emitted from a backlight, modulated in sub-pixels, and passes through a color filter.
FIG. 33 schematically illustrates a gamut of reproducible colors in a conventional liquid crystal display device. In FIG. 33, R, G, B, Ye, C, M, and W represent, respectively, red, green, blue, yellow, cyan, magenta, and white displayed by a pixel. Red, green, and blue correspond to sub-pixels of the liquid crystal display device and are also called primary colors. Yellow, cyan, and magenta are intermediate colors of the primary colors. The reproducible color gamut is shown as a vectorial sum of vectors to red, green, and blue with black (not shown) as reference, and white is at the center of the vectorial sum. The chromaticity of white is equal to that of black in FIG. 33 for the sake of simplification. A color within the reproducible color gamut may be displayed by setting the luminances of red, green, and blue sub-pixels to arbitrary values.
FIG. 34 illustrates chromaticities at which a pixel displays red (R), green (G), blue (B), yellow (Ye), cyan (C), magenta (M), and white (W) in a conventional liquid crystal display device. As illustrated in Table 1, the conventional liquid crystal display device has a reproducible color gamut that is 69% when measured by NTSC ratio, and has a color temperature of 6,600 K.
TABLE 1NTSC ratioColor temperature69%6,600 K
The color temperature is 6,600 K in the conventional liquid crystal display device described with reference to FIGS. 31 and 32. A higher color temperature is desired in some cases. For instance, while the standard color temperature according to NTSC is about 6,500 K, the Japanese generally prefer a high color temperature and color television sets for the Japanese market are set to 9,300 K (see, for example, Japan Broadcast Publishing Co., Ltd., “Broadcasting Technology Series 2: Broadcasting Formats” 1st Ed., Japan, Jan. 20, 1983, pp. 130-132). A high color temperature liquid crystal display device may be realized by using a backlight that is high in color temperature, namely, a backlight that is high in intensity in the short wavelength region of the visible light spectrum (see, for example, JP 2001-228322 A).
As disclosed in JP 2001-228322 A, a predetermined color temperature may be realized by using a given backlight. However, the inventors of the present application have discovered that simply switching to a given backlight shifts the color tone and accordingly lowers the display quality.
Specifically, in a three-primary color liquid crystal display device, simply employing a backlight that is high in intensity in the short wavelength region (hereinafter, referred to as “high color temperature backlight”) shifts the color tone and accordingly lowers the display quality as mentioned above.
Multi-primary color liquid crystal display devices have also been proposed in which yellow sub-pixels are added to red, green, and blue sub-pixels in order to expand the reproducible color gamut. If a multi-primary color liquid crystal display device uses the same backlight as in a three-primary color liquid crystal display device, the additional yellow sub-pixels give a displayed color a yellowish overtone, which makes the color temperature lower than in a three-primary color liquid crystal display device. A multi-color liquid crystal display device therefore needs to use a backlight that is high in intensity in the short wavelength region (i.e., high color temperature backlight) in order to achieve a color temperature equivalent to that of a three-primary color liquid crystal display device. In this case, too, simply employing a high color temperature backlight shifts the color tone and lowers the display quality.